1. Field of the Invention
Reverse biased silicon p-n junction photodiodes are widely used for applications which require the detection of intensity-modulated light in the spectral wavelength range of 0.2 to 1.1 microns. In recent years, the sensitivity of these photodiodes, which operate at room temperature, has increased to the point that they are now challenging the monopoly once held by the photo-multiplier in low-light-level sensing systems. Similarly, the need for highly sensitive photodetectors in the 3 to 5 micron region of the infrared spectrum is being fulfilled by photovoltaic indium antimonide devices which operate at 77.degree.K.
Both silicon and indium antimonide photodiodes produce an output which is proportional to the intensity of the incident radiation; therefore, the output of these devices is a true replica of the intensity-modulated input signal. However, the output of these devices is so small, especially in the case of indium antimonide photodiodes, that a low-noise preamplifier is generally used to boost the signal amplitude to a useful level.
An alternative approach has been used successfully with silicon photodiodes employed as sensors of light which is intensity-modulated at microwave frequencies. This technique, in which both photodetection and amplification take place within a single p-n junction, is called photoparametric amplification. The primary advantage of this technique is its improved noise performance.
2. Description of the Prior Art
Photoparametric operation of a photodiode was first predicted by Ahlstrom, Matthei and Gartner in July 1959 ("REV SCI INST," July, 1959, pp. 592-593). It was first demonstrated by Saito in 1962 and reported in "Proceedings of the IRE," November, 1962, pp. 2369-2370. Shortly thereafter, in September 1963, David E. Sawyer reported the successful operation of a negative resistance photoparametric amplifier in "Proceedings of the IEEE," September, 1963, page 1238. Detailed analyses were then carried out separately by Saito and Fugii and Penfield and Sawyer and reported respectively in "Proceedings of the IEEE", August, 1964, pp. 978-979 and "Proceedings of the IEEE," April, 1965, at page 340. They essentially predicted that, if high Q photodiodes are employed, the photoparametric amplifier should provide an amplified output with a signal-to-noise ratio nearly equal to that of the unamplified output of the same photodiode. This prediction was experimentally confirmed in 1966 by Grace and Sawyer, who utilized a specially fabricated silicon device with a p -v-n-n.sup.+ construction designed for excellent photodetection properties but also retaining a high Q for good parametric operation. This was reported in the IEEE TRANSACTIONS ON ELECTRONIC DEVICES, December, 1966, at pages 903 and 904. Roulston, in 1968, made a similar analysis for a photoparametric upconverter which he also verified and reported in the "IEEE Journal of Solid-State Circuits," December, 1968, pp. 431-440.
The devices reported in Sawyer and Roulston both operate with high frequency pumps ie, greater than 600 megahertz. Therefore these devices require microwave structure with the attended requirement of high precision components.